1. Field of the Invention
The present invention relates to a mechanical braking system for use on a vehicle. More particularly, the present invention relates to a mechanical braking system for use on a vehicle having an electric propulsion system and automatic retard speed regulation.
2. Related Art
An electric propulsion system for a vehicle, such as a traction vehicle, includes two electric traction motors coupled to a pair of rear wheels on opposite sides of the vehicle. The two electric motors are generally adjustable speed, reversible direct current (dc) motors and can operate so as to either propel or dynamically retard the traction vehicle.
In the propel mode, the two motors operate so as to adjust the speed of the vehicle. In the retard mode, or electric braking mode, the two motors behave as generators. Dynamic braking resistor grids are connected across the armatures of the respective motors in order to dissipate the electric power output of the motors during electric braking.
While in the retard mode, the goal of the two motors is to maintain a reference desired speed of the vehicle. For example, when the vehicle goes downhill, gravity will tend to accelerate the vehicle. In such a condition, the two motors will enter the electric braking mode to slow down the vehicle. Depending on the grade of road, however, it is possible that the electric motors will not be able to maintain the desired speed of vehicle. In addition, when the road condition is icy or wet, traction effort is reduced, and if one or both of the rear wheels becomes locked, the vehicle may enter a sliding mode. In this case, retard effort is reduced in order to avoid the sliding condition. Consequently, in some particular conditions of the road, the vehicle speed may increase, and the electric propulsion system cannot maintain the desired speed.
Under any such condition in which the electric propulsion system alone is not able to maintain the desired speed, an alarm signal may be output which alerts the driver of the vehicle that mechanical braking must be performed in order to maintain the desired speed of the vehicle. An example of an electric propulsion system for a traction vehicle which outputs such an alarm signal is disclosed in U.S. Pat. No. 4,495,449, the disclosure of which is incorporated herein by reference in its entirety.
Manually applying a mechanical braking system after such an alarm is received, however, requires that the driver of the vehicle be aware of the desired vehicle speed, and that the driver be able to apply the braking in an appropriate manner. Therefore, what is needed is a mechanical braking system that can be automatically applied so as to maintain a desired speed of a vehicle when the electric propulsion system of the vehicle is not able to maintain the desired speed by itself.